How to make my Windows Form app snap to screen edges?

Question

Anyone out there know how to make your .net windows form app sticky/snappy like Winamp so it snaps to the edges of the screen?

The target framework would be .NET 2.0 Windows Form written in C#, using VS08. I am looking to add this functionality to a custom user control, but I figured more people would benefit from having it described for the application and its main form.

Thank you.

Answer 1

This worked pretty well, works on multiple monitors, observes the taskbar:

  public partial class Form1 : Form {     public Form1() {       InitializeComponent();     }     private const int SnapDist = 100;     private bool DoSnap(int pos, int edge) {       int delta = pos - edge;       return delta > 0 && delta <= SnapDist;     }     protected override void  OnResizeEnd(EventArgs e) {       base.OnResizeEnd(e);       Screen scn = Screen.FromPoint(this.Location);       if (DoSnap(this.Left, scn.WorkingArea.Left)) this.Left= scn.WorkingArea.Left;       if (DoSnap(this.Top, scn.WorkingArea.Top)) this.Top = scn.WorkingArea.Top;       if (DoSnap(scn.WorkingArea.Right, this.Right)) this.Left = scn.WorkingArea.Right - this.Width;       if (DoSnap(scn.WorkingArea.Bottom, this.Bottom)) this.Top = scn.WorkingArea.Bottom - this.Height;     }   } 

Answer 2

The accepted answer only snaps the window after finishing the drag, whereas I wanted the form to continuously snap to the screen edges while dragging. Here's my solution, loosely based off the Paint.NET source code:

using System; using System.ComponentModel; using System.Drawing; using System.Runtime.InteropServices; using System.Windows.Forms;  namespace Whatever {     /// <summary>     /// Managed equivalent of the Win32 <code>RECT</code> structure.     /// </summary>     [StructLayout(LayoutKind.Sequential)]     public struct LtrbRectangle     {         public int Left;         public int Top;         public int Right;         public int Bottom;          public LtrbRectangle(int left, int top, int right, int bottom)         {             Left = left;             Top = top;             Right = right;             Bottom = bottom;         }          public Rectangle ToRectangle()         {             return Rectangle.FromLTRB(Left, Top, Right, Bottom);         }          public static LtrbRectangle FromRectangle(Rectangle rect)         {             return new LtrbRectangle(rect.X, rect.Y, rect.X + rect.Width, rect.Y + rect.Height);         }          public override string ToString()         {             return "{Left=" + Left + ",Top=" + Top + ",Right=" + Right + ",Bottom=" + Bottom + "}";         }     }      /// <summary>     /// A form that "snaps" to screen edges when moving.     /// </summary>     public class AnchoredForm : Form     {         private const int WmEnterSizeMove = 0x0231;         private const int WmMoving = 0x0216;         private const int WmSize = 0x0005;          private SnapLocation _snapAnchor;         private int _dragOffsetX;         private int _dragOffsetY;          /// <summary>         /// Flags specifying which edges to anchor the form at.         /// </summary>         [Flags]         public enum SnapLocation         {             None = 0,             Left = 1 << 0,             Top = 1 << 1,             Right = 1 << 2,             Bottom = 1 << 3,             All = Left | Top | Right | Bottom         }          /// <summary>         /// How far from the screen edge to anchor the form.         /// </summary>         [Browsable(true)]         [DefaultValue(10)]         [Description("The distance from the screen edge to anchor the form.")]         public virtual int AnchorDistance { get; set; } = 10;          /// <summary>         /// Gets or sets how close the form must be to the         /// anchor point to snap to it. A higher value gives         /// a more noticable "snap" effect.         /// </summary>         [Browsable(true)]         [DefaultValue(20)]         [Description("The maximum form snapping distance.")]         public virtual int SnapDistance { get; set; } = 20;          /// <summary>         /// Re-snaps the control to its current anchor points.         /// This can be useful for re-positioning the form after         /// the screen resolution changes.         /// </summary>         public void ReSnap()         {             SnapTo(_snapAnchor);         }          /// <summary>         /// Forces the control to snap to the specified edges.         /// </summary>         /// <param name="anchor">The screen edges to snap to.</param>         public void SnapTo(SnapLocation anchor)         {             Screen currentScreen = Screen.FromPoint(Location);             Rectangle workingArea = currentScreen.WorkingArea;             if ((anchor & SnapLocation.Left) != 0)             {                 Left = workingArea.Left + AnchorDistance;             }             else if ((anchor & SnapLocation.Right) != 0)             {                 Left = workingArea.Right - AnchorDistance - Width;             }             if ((anchor & SnapLocation.Top) != 0)             {                 Top = workingArea.Top + AnchorDistance;             }             else if ((anchor & SnapLocation.Bottom) != 0)             {                 Top = workingArea.Bottom - AnchorDistance - Height;             }             _snapAnchor = anchor;         }          private bool InSnapRange(int a, int b)         {             return Math.Abs(a - b) < SnapDistance;         }          private SnapLocation FindSnap(ref Rectangle effectiveBounds)         {             Screen currentScreen = Screen.FromPoint(effectiveBounds.Location);             Rectangle workingArea = currentScreen.WorkingArea;             SnapLocation anchor = SnapLocation.None;             if (InSnapRange(effectiveBounds.Left, workingArea.Left + AnchorDistance))             {                 effectiveBounds.X = workingArea.Left + AnchorDistance;                 anchor |= SnapLocation.Left;             }             else if (InSnapRange(effectiveBounds.Right, workingArea.Right - AnchorDistance))             {                 effectiveBounds.X = workingArea.Right - AnchorDistance - effectiveBounds.Width;                 anchor |= SnapLocation.Right;             }             if (InSnapRange(effectiveBounds.Top, workingArea.Top + AnchorDistance))             {                 effectiveBounds.Y = workingArea.Top + AnchorDistance;                 anchor |= SnapLocation.Top;             }             else if (InSnapRange(effectiveBounds.Bottom, workingArea.Bottom - AnchorDistance))             {                 effectiveBounds.Y = workingArea.Bottom - AnchorDistance - effectiveBounds.Height;                 anchor |= SnapLocation.Bottom;             }             return anchor;         }          protected override void WndProc(ref Message m)         {             switch (m.Msg)             {                 case WmEnterSizeMove:                 case WmSize:                     // Need to handle window size changed as well when                     // un-maximizing the form by dragging the title bar.                     _dragOffsetX = Cursor.Position.X - Left;                     _dragOffsetY = Cursor.Position.Y - Top;                     break;                 case WmMoving:                     LtrbRectangle boundsLtrb = Marshal.PtrToStructure<LtrbRectangle>(m.LParam);                     Rectangle bounds = boundsLtrb.ToRectangle();                     // This is where the window _would_ be located if snapping                     // had not occurred. This prevents the cursor from sliding                     // off the title bar if the snap distance is too large.                     Rectangle effectiveBounds = new Rectangle(                         Cursor.Position.X - _dragOffsetX,                         Cursor.Position.Y - _dragOffsetY,                         bounds.Width,                         bounds.Height);                     _snapAnchor = FindSnap(ref effectiveBounds);                     LtrbRectangle newLtrb = LtrbRectangle.FromRectangle(effectiveBounds);                     Marshal.StructureToPtr(newLtrb, m.LParam, false);                     m.Result = new IntPtr(1);                     break;             }             base.WndProc(ref m);         }     } } 

And here's what it looks like: